This invention is particularly applicable to aviation electrical power generation requirements that rely on state of the art electronic features, such as variable speed, constant frequency ("VSCF") electrical power generators used aboard military and commercial aircraft. However, it will be appreciated that this invention has broader applications wherever it is needed to provide a single phase or a multiple phase regulated voltage output from a static inverter. Such applications include industrial drives, power conditioning equipment, and the generation of controllable leading or lagging reactive current.
Each VSCF generator unit aboard an aircraft includes a three-phase power inverter paired with a propulsion engine so that each engine can be used to help fulfill the requirement of system redundancy to enhance operational safety. The aircraft's electrical power requirement is normally supplied by either a 24 or a 36-step wave inverter which provides from 20 to 50 kilovoltamps (KVA), three-phase, 115 VAC, operating at 400 Hz. As is known in the art, a 24-step wave inverter comprises, four, three-phase inverter bridges, and a 36-step wave inverter comprises six, three-phased inverter bridges. Each inverter bridge comprises six semiconductor power switching devices such as Gate Turn-Off (GTO) Thyristors, Insulated Gate Bipolar Transistor Pairs (IGBT), MOS-Controlled Thyristors (MCT) or other static switching devices. Each power switch is arranged and properly timed to provide three-phase AC voltages from a DC supply. The outputs of a equal number of inverter bridges are connected to the primary of an output transformer in either a delta or wye configuration. The secondary windings of the output transformers are interconnected in a series-add configuration which provide a multi-step waveform as discussed in U.S. Pat. No. 3,638,094.
A problem with the current design of VSCFs is that when a power switch in an inverter bridge develops a short circuit, the VSCF shuts off, and no AC power is delivered to the AC output bus. When this occurs, an auxiliary back-up power system on the aircraft switches on line to provide the necessary emergency electrical power for the aircraft to land safely. This back-up power system provides, what is commonly known in the industry as, fail-safe (FS) operation. However, no VSCF system currently provides a failed-operation (FO) redundancy safety system.